As one type of a wireless power transmission device, patent document 1 discloses a wireless power supplying device that detects displacement of an electric appliance, which is a power supplying subject, from the power factor of power supplied from a primary resonance circuit. The power supplying device adjusts the characteristics of the primary resonance circuit based on the power factor to limit decreases in the power supplying efficiency when displacement occurs.
In the power supplying device, if a power supplying operation is performed when there is a foreign matter, the foreign matter may be heated. It is thus desirable that such a foreign matter be detected to stop the power supplying operation.
Patent document 2 discloses a wireless power supplying device that includes a detection coil for detecting metal in addition to a primary coil for supplying power. The power supplying device excites and drives the primary coil to supply an electric appliance with power. Further, the power supplying device excites and drives the detection coil to detect the electric appliance or a foreign matter, such as metal, based on changes in the impedance of the detection coil.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2012-130173
Patent Document 2: Japanese Laid-Open Patent Publication No. 2006-230129